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PAM-4 Transmission Over Large-Core Plastic Optical Fiber Using the OptSim DSP Library for MATLAB
Tool Used: ModeSYS
Due to its relaxed alignment tolerances and low cost, step-index (SI) plastic optical fiber (POF) is an attractive technology for short-distance data interconnects. Furthermore, PAM-4 modulation with DSP equalization at the receiver can help overcome the inherent bandwidth limitations of POF at Gb/s data rates [1],[2].
Based partially on work presented in [1] and [3], Figure 1 illustrates the topology in ModeSYS that implements end-to-end 1-Gb/s PAM-4 transmission over 1-mm diameter SI-POF. ModeSYS simulates the fiber using a computationally efficient model based on the Gloge power-flow equation, while MATLAB co-simulation models the transmitter and receiver DSP using functions from the OptSim DSP library for MATLAB. The transmitter generates an electrical PAM-4 signal with Nyquist pulse shaping and pre-emphasis filtering, and the receiver DSP uses an LMS equalization algorithm to recover the transmitted symbols and analytically estimate the BER.
Figure 2 depicts the optical eyes at the POF input and output, showing eye closure due to intermodal dispersion and mode coupling. Figure 3(a) illustrates the recovered eye diagram after the receiver’s analog-to-digital conversion and equalization, which has compensated for the eye closure, while Fig. 3(b) depicts the analytically estimated BER in comparison with an ideal PAM4 reference curve. The BER of 2.35 × 10-4 is below an FEC threshold of 3.7 × 10-3 [4],[5].
Figure 1. Topology for simulating PAM4 transmission over SI-POF.
(a) (b)
Figure 2. Simulated POF input (left) and output (right) eyes, showing the effects of intermodal dispersion and mode coupling.
Figure 3. (a) Equalized eye at the receiver demonstrating compensation of eye closure. (b) Analytical BER estimate in comparison to ideal reference curve.
References
1. R. Kruglov, S. Loquai, C.-A. Bunge, M. Schueppert, J. Vinogradov, and O. Ziemann, “Comparison of PAM and CAP modulation schemes for data transmission over SI-POF,” IEEE Photonics Technology Letters, vol. 25, no. 23, pp. 2293-2296, December 1, 2013.
2. F. Breyer, S. C. Jeffrey Lee, S. Randel, and N. Hanik, “Comparison of OOK- and PAM-4 modulation for 10 Gbit/s transmission over up to 300 m polymer optical fiber,” OFC/NFOEC 2008, paper OWB5, 2008.
3. R. Kruglov, J. Vinogradov, S. Loquai, O. Ziemann, C.-A. Bunge, T. Hager, and U. Strauss, “21.4 Gb/s discrete multitone transmission over 50-m SI-POF employing 6-channel WDM,” OFC 2014, paper Th2A.2, 2014.
4. F. Karinou, C. Prodaniuc, N. Stojanovic, M. Ortsiefer, A. Daly, R. Hohenleitner, B. Kögel, and C. Neumeyr, “Directly PAM-4 modulated 1530-nm VCSEL enabling 56 Gb/s/l data-center interconnects,” IEEE Photonics Technology Letters, vol. 27, no. 17, pp. 1872-1875, September 1, 2015.
5. A. Yekani, M. Chagnon, C. S. Park, M. Poulin, D. V. Plant, and L. A. Rusch, “Experimental comparison of PAM vs. DMT using an O-band silicon photonic modulator at different propagation distances,” 2015 European Conference on Optical Communication, paper Mo.4.5.1, 2015.